Seven years ago today, it was hard to believe how many people were on the streets trying to prevent the Iraq war. There was anger and foreboding, but also a feeling of exhilaration. Surely a march this big would have some effect. As we know, it didn't.

Yet the numbers on that 15 February 2003 march, the biggest demonstration ever held in Britain (joined by more than 10 million people in over 60 countries in what became the largest worldwide demonstration in history), do matter. Because what they pointed to was something unique. For every regular protester, there were a vast number of people who'd never marched before.

So was the protest a failure? Given that we know now that Tony Blair had decided for war, any number of millions might not have deterred him. But the protesters have succeeded in making opposition to war a part of mainstream political debate. Not just in parliament, where the vote for war was won by only 12 MPs, but in the traditionally conser­vative press. The Daily Mail pointed to the anti-war demonstrators gathered for Blair's appearance before the Chilcot inquiry, and a poll showing that 80% of people polled thought Blair was lying, to state the country now shared the ­sentiments of the protesters.

The protest also shattered the myth of political apathy. While whole series of demonstrations have been largely ignored by the media, in a bedrock of growing public disillusionment, even the most unsympathetic could hardly fail to take note of the size of that turnout.

No movements succeed overnight. The civil rights movement in the United States eventually led to the end of segregation, and the Suffragettes in Britain got the vote. In the case of Iraq, as far back as 2002 Barack Obama declared his opposition to the war at a rally in Chicago, just as President Bush and Congress announced their agreement on the joint resolution authorising it. Obama and Hillary Clinton jockeyed for the anti-war position, even if he felt he needed to balance it with supporting a surge in Afghanistan. That was possible because opposition to war has become an accepted part of US political debate.

The effects and lessons of that protest look especially pertinent today. Britain still finds itself in one war wrought in the aftermath of 9/11, and at the weekend the hostilities in Afghanistan intensified. Luckily, Blair is no longer prime minister or – going by what he said 58 times to the Chilcot inquiry – Britain would be well on the path to war with Iran, too. But will people take to the streets again if moves against Iran escalate?

Opposition to war in Iran is widespread even within the establishment. In 2008, Admiral William Fallon, then head of US central command, resigned in part because of his opposition to threats against Iran. Obama was able to make opposition to aggression a main plank of his campaign, even if in practice he has fallen short of his promises. Several Nobel prizewinners, including the Iranian Shirin Ebadi, oppose such an attack, as do the Non-Aligned Movement of more than 100 countries. Numerous polls show nearly 70% of Americans oppose it too.

And the legacy of 2003 is that today, many – on both the right and left – feel that it would be impossible for any government, even one convinced of a case for a just war, to carry the nation with it.

Trust between the people and politicians has broken down, but the demon­strations should give us reasons for hope, not disillusionment. Most major political advances in the UK and abroad have come through protest, whether it is women's rights or civil rights. Wars corrode our political system. But protest is the engine of democracy, and may again be our best, if not only, hope.